Tunable Microfiber Nonlinear Phenomenon

The preparation of low-loss micro-nanofiber has been realized in 2003. Since then, nano/micro-fiber has drawn much attention, due to its unique optical properties. Particularly, in the field of nonlinear optics, nano/micro-fiber has become an advanced research hotspot, for its enhanced nonlinear coefficient. It is caused by the reason that its mode field diameter is close to the optical wavelength, which increases the optical field density and decreases nonlinear threshold. In this dissertation, the four-wave mixing and the producing process in nano/micro-fiber would be simulated and analyzed. By the means of external control, the tunable characteristics in nonlinear process are clarified and the producing process of refractive index controlled second harmonics is realized.The specific work is as following:1. The realization of strain modulated four-wave mixing effect in an elliptical microfiber. We analyzed the dispersion and nonlinear effect of an elliptical microfiber with birefringence. By choosing suitable size, we observed the frequency shift of the strain modulated four-wave mixing effect in simulation. For elliptical microfiber with ellipticity 3:2, the maximum shift of the signal is 72.35 nm within 30 mε. This characteristic can be used to design strain sensor. The average sensitivity of 2.42 pm/με is achieved within 30 ms and it would be optimized further to 9.72 pm/με when a smaller strain range 5mε is permitted. This strain sensor has simple construction, low price, and high sensitivity, which can be used as wavelength-tuning entangled photon source.2. The realization of temperature controlled four-wave mixing effect. The temperature will affect the optical property of microfibers, because of the thermo-optic effect. By heating the microfiber from 25 to 100 degree centigrade, we analyzed its dispersion variation. We can see that there is only a little change of dispersion curve, which causes the zero dispersion wavelength shifts 0.04nm/℃. However, it will cause considerable change in the four-wave mixing process in a microfiber with a suitable diameter, and can be measured by spectrograph. This temperature controlled four-wave mixing effect can be also used as a temperature sensor and a wavelength-tuning photon source. The temperature sensitivity can be as high as 282 pm/℃, which is dozens of times higher than temperature sensor with common photonic crystal fiber.3. The realization of refractive index controlled four-wave mixing effect. In this part, we applied two methods. Firstly, we used liquids, such as water, methyl alcohol, and ethyl alcohol, to uncover the waist of microfiber. By changing the cladding material, we analyzed the influence of different refractive index to four-wave mixing effect in simulation. Within the normal dispersion regime of 1064nm pump micro fiber, the remote signal is realized. Meanwhile, the diameter of 1064nm matching microfiber is increased by changing cladding material. For 1.7μm microfiber, with contrastive analysis of water and methyl alcohol, which have similar refractive index, we acquired 400nm shift of optical signal. This means the refractive index of cladding layer has great influence on the control of four-wave mixing effect. Secondly, we applied different thickness of Teflon cladding layer to realize multi-clad effect. With different thickness, the changing of micro fiber dispersion is different. The zero dispersion wavelength shifts more than 400nm from thickness of 100nm to 500nm. In this sense, the thick ness of cladding layer has significant influence in the tuning of four-wave mixing effect.The producing process of refractive index controlled harmonic wave. We still used liquids and Teflon cladding layer microfiber to regulate and control the matching method of harmonic generation. To enhance the regulation effect, the Teflon cladding layer should be as thick as possible, such that it can be taken as the only cladding layer. With 1064nm pump light, the matching diameter of microfiber can increase 50% to 80%. We can increase the matching diameter of microfiber by changing the solution concentration, and meanwhile adjust the matching condition of harmonic continuously. This part has been demonstrated by experiment. We added ethyl alcohol to water cladding layer to control the refractive index of liquids, such that the matching diameter of second harmonics increases, In experiment, we observed the second harmonics grow out of nothing and then disappear, In this sense, we realized the external control of the matching method of second harmonics.In summary, we studied nonlinear process, four-wave mixing and harmonic generation in external controlled fiber theoretically and experimentally. A few kinds of external control mechanism have been analyzed. We believed that this research will be instructive to the development of tunable frequency conversion in the future.